A prism sheet, also called a brightness enhancement film (BEF), is an important component, which occupies 37% of the costs of an LCD backlight module. A prism sheet is a film material, which makes diffused light concentrated within a certain range and emitted by utilizing the law of emission and refraction.
In general, a prism sheet's lower surface is a planar surface, and its upper surface is in a wave structure, that is, it comprises a plurality of protrusions in parallel with one another. Referring to FIG. 1, the prism sheet comprises: a lower surface 10, and an upper surface 20, and the upper surface 20 has a plurality of protrusions 21 thereon in parallel with one another. Herein, the axial cross-sections of the protrusions 21 are isosceles triangles, and moreover, the protrusions 21 are arranged in parallel along a horizontal direction. Certainly, the protrusions 21 on the prism sheet also may be arranged in parallel along a perpendicular direction.
In an LCD backlight module, light generated by a plurality of light sources, after being diffused through a diffusion plate, enters into a prism sheet, in which light is emitted in a certain degree of concentration under the effect of the protrusions parallel with one another on the prism sheet, thereby increasing the brightness of the backlight module within a particular range of viewing angles.
At present, an LCD backlight module comprises a plurality of light sources therein, and these light sources are usually arranged along a set direction. for example, a horizontal direction, a perpendicular direction, or a rectangular array direction. When it is required to concentrate the light emitted from the plurality of light sources in a horizontal direction, a prism sheet having protrusions arranged in parallel along the horizontal direction on its upper surface is used; when it is required to concentrate the light emitted from the plurality of light sources in a perpendicular direction, a prism sheet having protrusions arranged in parallel along the perpendicular direction on its upper surface is used; when it is required to implement light concentration in both of the two directions, two types of prism sheets described above are used simultaneously, which can be specifically referred to FIG. 2, in which two prism sheets are used.
Prism sheets for two directions are required to work in cooperation in order to achieve light concentration in two directions, and in result not only the assembling processes of an LCD backlight module are increased, but also the light concentration effect will not be changed when movement of the two prism sheets occurs, and further the performances of the LCD backlight module is affected. Therefore, the existing backlight module needs enough space to accommodate the two prism sheets, which makes the backlight module relatively thick and reliability not high.